The present invention concerns laser plasma generation method using a target material and its system. The target material is at least one of liquid material, solid material and chemically inert gas element. The target material of the gas element is in a liquid or solid state at a low temperature, or in a low temperature gas state with a vapor density nearly equal to a density in the liquid state. And the method or system focuses and irradiates pulsed laser beam as the main laser beam onto the target to heat the target material, generating high-temperature high-density plasma.
Especially, it concerns practical laser plasma generation method and its system which can generate stable high-temperature high-density plasma continuously without giving any damages on x-ray optical components and x-ray detectors mounted around the plasma by the fast debris from the target.
Until now, in such laser plasma generation system the high-temperature high-density plasma is formed by focusing and irradiating a pulsed laser beam having a high-peak power into a spot of less than 100 μm in diameter on the target material of solid density. Application equipment of this x-ray has been used practically since 1970's, because this plasma lump radiates the x-ray of high-brightness.
Any of the target materials of solid density used here is solid material mainly composed of metals such as copper (Cu), aluminum (Al) and gold (Au). Therefore, there is a problem that molecules and atoms of the target material are evaporated by laser heating and deposit on the internal surface of the chamber wall, the surface of x-ray mirrors which collects the radiant x-ray, and the entrance window of x-ray detector, etc. Accordingly, these surfaces must be cleaned periodically.
In order to solve such problem, A. L. Hoffman et al (Vacuum Science and Technology B3(1), pp. 258, 1985) proposed the technology which prevents the dispersion of ejected solid debris by equipping a mechanical shutter in the space to the surface of the x-ray mirror.
N. Kandaka et al, Jap. J. Appl. Phy. 37, L147(1998), tried to stop the foregoing evaporated gas molecules to flow to the x-ray mirror by means of placing a gas cell that forms a dense buffer (collision) gas region. However it is not possible to take a sufficient degree of solid angle which collects the x-ray because the distance to the x-ray mirror is generally large.
On the other hand, Japanese Unexamined Patent Application (JP-A) No. H11-250842 has been disclosed for the purpose of obtaining laser plasma light source with high x-ray conversion efficiency and less scatters of debris. In this proposal, the dent formed on the surface of solid target is irradiated with the laser beam for ablation. By this irradiation, the surface part minute of internal wall of the dent is gasified and then the emitted gas is focused in the space inside the dent, for example in the vicinity of the central part of the exit to form the high-density lump. The high-temperature plasma is obtained by irradiating the pulsed laser to heat the high-density lump.
In this method, it is assumed that the generation of the debris is little because only the surface part minute is gasified and it is also assumed that the high-density lump formed at the exit of the dent restrains the emission of the fine debris to the outside.
As described above, when the plasma is generated by using the target material of liquid or solid, the strong shock wave arises and propagates into the inside of target material, because the local rapid pulse-like pressure rises with the generation of the plasma. In the meantime, the fine debris which have been produced by crushing the target material by the shock wave are discharged to the outside direction after the plasma lump consisting of electrons and ions diffused into the vacuum space.
There is a problem that the fine debris collides with the nearby surface, giving the mechanical damage when their speed, namely kinetic energy density, is large.
In conventional laser plasma generation systems which solve such problem, the method using the mechanical shutter does not only have a problem that ultrahigh speed rotation of the shutter is required, but also a problem that the acceptance solid angle for the x-ray is much limited. And, in the example of the gas cell method the block effect by the gas is too weak to stop the diffusion of fast debris which has a large momentum.
And, the practicability is questionable for the example of forming the high-density lump in the space in the dent and then irradiating it with the pulsed laser beam.
That is, in the focusing and irradiating of the pulsed laser beam, it is an important prerequisite that the target material which spouted from the surface part minute of the solid target converges according to the three dimensional effect by the dent, and forms temporarily a density-compressed gas lump.
However, in order to generate the plasma at a high-repetition rate it is necessary to form the dent beforehand at the right position precisely so that the laser beam is projected accurately on the target surface.
In addition, it is necessary to forms the uniform vapor flow to the focus spot from the wide parts in the surface of the dent so that the flows reach the focus spot simultaneously. The dent whose cross-sectional view has a hemi-spherical shape is exemplified in order to realize the process above described.
That is to say, it is necessary to irradiate the laser beam in such a way to almost uniformly heat full face of the hemi-sphere inside. And, the spherical structure is necessary in such a way that the vapor flows from the surface of the dent concentrate at the center of the exit, but its realization seems to be difficult. And, it has been described in the proposal that a very short pulsed high-peak power laser beam should be used for ablation because only the surface part minute of the dent needs to be gasified.
However, it is necessary that the target material surface must absorb the laser energy necessary for giving the kinetic energy to the material gasified in addition to vaporizing energy, in pico-second period. Thus it is not inevitable that a large pressure impulse with a high-peak power arises and this impulse likely drives a strong shock wave.
The shock wave crushes the solid material to generate the debris. Therefore, the inhibition of the debris generation can not be expected, even if the dent is used for gas compression. In addition, the high-density lump exists only for a short period compared to the duration of the debris generation and it is not possible to stop the debris scatter to the outside of the dent when the density of the lump is insufficient and unstable.